The present invention relates to a process for preparing crystals of anhydrous sodium sulfide.
Currently commercially available sodium sulfides include crystalline sodium sulfides containing water of crystallization (such as Na.sub.2 S.multidot.9H.sub.2 O, Na.sub.2 S.multidot.6H.sub.2 O, Na.sub.2 S.multidot.5.5H.sub.2 O, Na.sub.2 S.multidot.5H.sub.2 O and the like) prepared by cooling or concentrating an aqueous solution of sodium sulfide to cause precipitation; water-containing non-crystalline sodium sulfides prepared by solidifying into pellets, flakes, chips or the like the whole of a hot aqueous solution of sodium sulfide having a concentration of about 60%; etc. However, these sodium sulfides, which have a water content of 30% or more, suffer the drawbacks of being significantly deliquescent and easily oxidizable. Furthermore, the chemical reaction using such sodium sulfide poses the problem that the water present in the sodium sulfide may induce the undesirable side reaction or may shift the direction of progress of reaction. In view of these problems, there is a demand for anhydrous sodium sulfides free of such defects.
Conventional processes for preparing anhydrous sodium sulfide by dehydration of hydrated sodium sulfide include, for example, those comprising dehydrating hydrated sodium sulfide under reduced pressure by heating at a temperature at which the hydrate is partially fused. However, fusion of hydrated sodium sulfide gives a highly viscous mass which firmly adheres to a reactor, becoming difficult to stir or withdraw.
Also known is a process comprising heating hydrated sodium sulfide (Na.sub.2 S.multidot.9H.sub.2 O) without stirring under specific conditions including a reduced pressure of 1 torr and gradually elevating the temperature to 800.degree. C. without melting to achieve forced dehydration (U.S. Pat. No. 2533163). Yet this process is not suitable for use because it requires heating at an exceedingly high temperature. Moreover, the anhydrous sodium sulfide prepared by forced dehydration of hydrate assumes the skeletal crystal form retaining the original crystal form of hydrate, and is consequently great in specific surface area, deliquescent and readily oxidizable.
An object of the present invention is to provide a simplified process for preparing crystals of anhydrous sodium sulfide which are unlikely to deliquesce and to oxidize and easy to handle.
Other objects and features of the invention will become apparent from the following description.